A difficult obstacle associated with the exploration and production of oil and gas is management of significant ocean currents. These currents can produce VIV and/or large deflections of tubulars associated with drilling and production. VIV can cause substantial fatigue damage to the tubular or cause suspension of drilling due to increased deflections. Various types of VIV suppression devices, for example helical strakes and fairings, can be attached to the tubular in an effort to suppress the effects of VIV on the tubular. While helical strakes, if properly designed, can reduce the VIV fatigue damage rate of a tubular in an ocean current, they typically produce an increase in the drag on the tubular and hence an increase in deflection. Thus, helical strakes can be effective for solving the vibration problem at the expense of worsening the drag and deflection problem.
Another solution is to use fairings as the VIV suppression device. Typical fairings have a substantially triangular shape and work by streamlining the current flow past the tubular. A properly designed fairing can reduce both the VIV and the drag. Fairings can be made to be free to weathervane around the tubular in response to changes in the ocean current. Typically, fairings are applied to tubulars with a need for very low drag. Short fairings can be applied to a tubular to reduce both VIV and drag, but sometimes a further reduction of drag is required.
An option for achieving very low drag is to use longer fairings, i.e. fairings with longer chords. Longer chords help further streamline the flow in a more gradual fashion. There are several issues, however, associated with the use of long chord fairings. One issue is the need to keep the fairing aligned with the flow of water. While short fairings can reduce VIV substantially without fully aligning themselves with the flow, they experience an increase in drag if they do not align properly with the incoming flow. This increase in drag is even greater for longer chord fairings and, since longer chord fairings are selected mostly for their low drag characteristics, it is important that they align properly with the incoming flow.
A second issue associated with long chord fairings is the difficulty in quickly and securely installing these devices. Since they are larger, they are both more cumbersome to handle and will experience larger forces on them normal to the flow direction.
A third issue associated with long chord fairings is the cost of fabrication. Short fairings are typically more expensive than helical stakes, and long chord fairings are even more expensive. Thus, there is a concern that long chord fairings will be prohibitively expensive to use.
A fourth issue associated with long chord fairings is the tendency of long chord fairings to experience a plunge-torsional instability. This instability is also commonly known as flutter, and results in large motions of the tubular due to rotations of the fairings that are synchronized with the motion of the tubular.